Frequency reuse is one of methods for increasing channels per unit area in a cellular system. The intensity of an electric wave becomes weak gradually as the distance becomes long. Accordingly, since there is little interference between electric waves in a place away from a given distance, a single frequency channel can be used therein. In this way, one frequency can be used simultaneously in various zones. This could increase subscriber capacity. Such efficient use of the frequency will be referred to as frequency reuse. A unit for dividing zones from one another will be referred to as a cell (mobile communication cell), and frequency channel conversion between cells for maintaining communication will be referred to as handoff. In an analog cellular mobile communication system, the frequency reuse technique is necessarily required. A frequency reuse rate is one of parameters representing frequency efficiency in a cellular system. The frequency reuse rate is a value obtained by dividing the number of cells (sectors) which simultaneously use the same frequency in a multi-cell structure by a total number of cells (sectors) of the multi-cell structure.
A frequency reuse rate of a 1 G system (for example, AMPS) is smaller than 1. For example, for 7-cell frequency reuse, a frequency reuse rate is 1/7. A frequency reuse rate of a 2 G system (for example, CDMA and TDMA) has been more improved than that of the 1 G system. For example, in a GSM where FDMA and TDMA are used in combination, a frequency reuse rate reaches 1/4 to 1/3. In case of a 2 G CDMA system and a 3 G WCDMA system, since a frequency reuse rate can reach 1, spectral efficiency is increased and the network arrangement cost is reduced.
When all sectors within one cell and all cells within one network use the same frequency, a frequency reuse rate of 1 can be obtained. However, obtaining a frequency reuse rate of 1 in a cellular network means that edge users of cells have signal receiving throughput reduced by interference from neighboring cells.
In an OFDMA system, since a channel is divided in a sub-channel unit, signal transmission is performed on the sub-channel and all channels are not used unlike the 3 G (CDMA 2000 or WCDMA) system. Using this feature, throughput of cell center users and throughput of cell edge users can be improved at the same time. In this case, the edge region of the cell and the center region of the cell are not defined absolutely. Generally, since the center region of the cell is close to a base station, it means a region where users relatively safe against co-channel interference from neighboring cells are located. Accordingly, inner users located at the center of the cell can use all possible sub-channels. On the other hand, the edge region of the cell is a region where users relatively susceptible to inter-cell interference are located. The users at the edge of the cell use a frequency reuse factor (FRF) not 1, and do not use same frequency partition. Accordingly, at the edge of neighboring cells, a frequency is allocated in such a manner that respective cells use different sub-channels. This frequency allocation is called fractional frequency reuse (FFR).